An impeller is a rotating component that includes a hub and at least one blade. In operation, the impeller is used to accelerate and/or pressurize a fluid. More particularly, an impeller converts the rotary mechanical energy of a drive (e.g., a motor, etc.) into kinetic energy (flow) and potential energy (pressure) of a fluid being acted upon. Impellers are used in various types of equipment, including pumps, water jets, agitated tanks, washing machines, and vacuum cleaners, to name but a few.
The impeller is designed to enable a pump, etc., to achieve certain performance characteristics, such as a certain mass flow rate, pressure ratio, and/or efficiency. Device performance is ultimately a function of the operating conditions (e.g., inlet pressure, temperature, fluid density, etc.) as well as geometrical parameters of the impeller (e.g., hub diameter, blade geometry, etc.).
Impeller blades often have a very complex blade geometry intended to optimize hydrodynamic efficiency or meet other design criteria. Furthermore, the structure of impeller blades can vary dramatically as a function of intended application. Consider, for example, an airplane propeller blade or mixer blade. These blades tend to be relatively long in span and short in chord length. Contrast those blades with a screw-type impeller blade (e.g., Archimedes screw, etc.) having a single helical vane that exhibits a significant degree of wrap around the central hub. These screw-type blades are relatively short in span and long in chord length.
There are some specialized applications in which the impellers might have additional design requirements, such as an ability to expand and collapse. One such application is the percutaneously-inserted blood pump.
A blood pump is a cardiac-assist device that is useful as an intervention for some patients who have acute heart failure or who are at risk of developing it. An effective cardiac assist device assumes some of the heart's pumping function, thereby unloading the heart and enabling it to recover. The blood pump is typically intended as a temporary measure, usually in operation for less than a week.
Percutaneously-inserted blood pumps are designed to be inserted into a patient using a minimally-invasive procedure. These blood pumps are usually inserted via established cath-lab techniques, such as by inserting the blood pump into a peripheral vessel (e.g., femoral artery, etc.) and advancing it to the ascending aorta or the heart (e.g., Seldinger, etc.). To be percutaneously inserted into a peripheral vessel, a blood pump must be quite small. In particular, it is desirable for these blood pumps to have a 12-French (4 millimeter) or smaller catheter. This places a severe constraint on the size of the impeller blades and, hence, the amount of blood that the device can pump.
In an attempt to address this size constraint, the “expandable” blood pump has been proposed. This type of pump, which is suitably small for percutaneous insertion, includes an impeller that expands once in place within the heart or larger vasculature nearby. The blade span attained by the expanded impeller is greater than is otherwise possible for a non-expandable impeller (that is also percutaneously inserted). As a consequence, the expandable impeller can pump more blood per revolution and operate at a lower rotational speed. Most expandable blood pumps use one of several different implementations of the expandable impeller: inflatable impellers, pivoting impellers, or foldable impellers. Some examples of prior-art blood pumps that use these types of impellers are discussed below.
U.S. Pat. No. 6,981,942 discloses a percutaneously-insertable blood pump having an inflatable housing and an inflatable impeller, which includes an inflatable hub and a single blade-row of inflatable blades. The housing is attached to a long sheath that couples the pump (ultimately sited in/near the heart) to extracorporeal elements, such as a motor and source of pressurized air. A drive shaft that couples the impeller to the motor and inflation tubes for inflating the housing and impeller are disposed in the sheath.
U.S. Pat. No. 5,749,855 discloses a percutaneously-insertable blood pump having a pivoting impeller. The impeller comprises a single blade row of two blades that are surrounded by an expandable cage. A drive cable extends from an extracorporeal motor to the distal end of the cage. In the absence of an applied, axially-directed force, the cage and impeller remain in a collapsed state.
The drive cable is designed so that its inner part is movable relative to its outer part. As the inner part of the drive cable is drawn in the proximal direction by an axially-applied force (e.g., by a medical practitioner tugging on the inner part), relative movement between the inner and outer parts of the drive cable expands the cage and pivots the blades into a deployed state. The deployed propeller can then freely spin within the expanded cage.
U.S. Pat. No. 6,533,716 discloses a percutaneously-insertable blood pump having a foldable helical rotor. The rotor consists of a helical frame, which is embodied as a helically-twisted segment of Nitinol wire. Both ends of the helically-twisted segment are coupled to an elastic band that lies along the axis of rotation of the helical frame. A surface of the rotor “blade” is formed from a membrane that extends between the helical frame and the centrally-disposed elastic band. The membrane is formed from a spongy, woven tissue.
The helical rotor is in a collapsed state for insertion into the vascular system. In this state, a tube overlies the helical frame and forces it into an elongated configuration along the central axis. The centrally-disposed elastic band is under maximum tension and the covering membrane is compressed. When the covering tube is withdrawn, the elongated Nitinol wire contracts axially and assumes the helical shape. As this occurs, the elastic band contracts and the membrane forms a smooth surface that functions as the surface of the rotor.
U.S. Pat. No. 4,753,221 discloses a percutaneously-insertable blood pump that includes attributes of both inflatable and foldable impellers. This blood pump comprises a catheter, the distal end of which is formed from a flexible material that is capable of expanding. Blades, which are disposed in a single blade row, are formed from an elastic material and are disposed in the catheter at the flexible region. When the catheter is in a delivery or collapsed state, the blades are “bent over,” substantially parallel to the rotational axis of the pump. To deploy the blades, the distal end of the catheter is enlarged by inflating a balloon that couples to the exterior of the catheter. As the distal end of the catheter expands, the blades unfold into an operational position wherein they extend orthogonally to the rotational axis.
U.S. Pat. No. 4,919,647 discloses a percutaneously-insertable blood pump having a catheter to which four foldable impeller blades arranged in a single blade row are coupled. The blades are formed of an elastic material and are biased to naturally project radially outward. The blades are disposed in the distal end of a catheter, which has a cup-shaped form and is made from an expandable material. For insertion into a patient, the impeller blades and the cup-shaped portion are contracted radially inward, such as by placing the catheter within a tubular guide. When the guide is removed, the blades and the cup-shaped portion expand.
U.S. Publ. Pat. Appl. No. 2008/0114339 discloses a percutaneously-insertable blood pump having an impeller with foldable blades arranged in a plurality of blade rows. This reference discloses that it is difficult to fold a long helical blade that exhibits a substantial amount of wrap around the central hub. To address this problem, the reference discloses that a long blade should be segregated into two, three or perhaps more shorter blades that are arranged (i.e., spaced apart) into a like number of blade rows.
Although impeller design is a well-understood discipline, the expandable impeller, especially in the context of a blood pump, raises a variety of design challenges. In particular, and among any other issues, careful consideration must be paid to the structural adaption of the impeller that enables it to expand/collapse and the manner in which expansion/collapse is actuated. These issues are important because they typically affect the structural configuration of the surrounding pump structure (e.g., pump housing, etc.) and the way in which the impeller is integrated in the surrounding structure.
It is notable that even though the patent literature is replete with expandable blood pumps, including those discussed above, not one of them is currently in use. A need therefore remains for an expandable impeller that can be used in percutaneously-insertable blood pumps, among other applications.